The present invention relates to conditioning of a polishing pad employed in chemical-mechanical polishing (sometimes referred to as "CMP"). More particularly, the present invention relates to conditioning a polishing pad using a conditioning reagent that includes at least one of hydrofluoric acid, buffered oxide etch composition and potassium hydroxide.
Chemical mechanical polishing (CMP) typically involves mounting a wafer faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or is in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In semiconductor wafer fabrication, this technique is commonly applied to polish various wafer layers such as dielectric layers, metallization, etc.
Unfortunately after polishing on the same polishing pad for over a period of time, the polishing pad suffers from "pad glazing." As is well known in the art, pad glazing results when the particles eroded from the wafer surface along with the abrasives in the slurry tend to glaze or accumulate over the polishing pad. A glazed layer on the polished pad typically forms from eroded film and slurry particles that are embedded in the porosity or fibers of the polishing pad. Pad glazing is particularly pronounced during planarization of an oxide layer such as silicon dioxide layer (hereinafter referred to as "oxide CMP"). By way of example, during oxide CMP, eroded silicon dioxide particulate residue accumulates along with the abrasive particles from the slurry to form a glaze on the polishing pad. Pad glazing is undesirable because it reduces the polishing rate of the wafer surface and produces a non-uniformly polished wafer surface. The non-uniformity results because glazed layers are often unevenly distributed over a polishing pad surface.
One way of achieving and maintaining a high and stable polishing rate is by conditioning the polishing pad (hereinafter referred to as "pad conditioning") on a regular basis, i.e. either every time after a wafer has been polished or simultaneously during wafer CMP. FIG. 1 shows a side-sectional view of a cross-section of a polishing pad 102 undergoing conditioning according to conventional methods of pad conditioning. A surface of polishing pad 102 that is employed for oxide CMP has deposited on it a glazed oxide layer 104 resulting from pad glazing described above. During pad conditioning, a stream of water 106 is introduced over the polishing pad and a particle 108, for example, is dislodged from glazed layer 104 by mechanical action, described below in detail, to produce dislodged particles 108'. Furthermore, loose oxide particles 110 may also be found on the surface of polishing pad 104 as eroded oxide or slurry residue from CMP.
During pad conditioning, a conditioning arm or an abrasive disk (both not shown to simplify illustration) having an abrasive portion contact polishing pad 102, which may be rotating or in an orbital state. A pneumatic cylinder (not shown to simplify illustration) may then apply a downward force on the conditioning arm or abrasive disk such that the abrasive portion contacts and engages with a substantial portion of polishing pad 102. During pad conditioning, the conditioning arm typically sweeps back and forth across polishing pad 102 like a "windshield wiper blade" from one end of the polishing pad to another. In the case where pad conditioning is facilitated by the abrasive disk, the abrasive disk typically moves in a radial direction and out of an inner point and an outer point of polishing pad 102. By this mechanical action of the conditioning arm or abrasive disk, the abrasive portions attempt to break up and remove the glazed or accumulated particles coated on the polishing pad surface.
Unfortunately, the mechanical action of conventional pad conditioning processes are ineffective to completely remove the glazed layer. The sweeping action of the conditioning arm or the radial movement of the abrasive disk across the polishing pad distributes the mechanical action of the abrasive portions almost uniformly throughout the polishing pad surface. Such mechanical action is not forceful enough to remove the glazed layer from those regions of the polishing pad where the glazed layer is thicker and/or compacted to a greater extent than that on other regions of the polishing pad. The problems of lower material removal rate and nonuniform polishing, therefore, still persist albeit to a lesser degree. Furthermore, the unremoved portions of the glazed layer prevent the formation of grooves on the polishing pad during pad conditioning. As is well known to those skilled in the art, polishing pad grooves facilitate the polishing process by creating point contacts between the wafer surface and the pad, increase the pad roughness and between the wafer surface allow more slurry to be applied to the substrate per unit area. Thus, due to the presence of the glazed layer and absence of grooves on the polishing pad, a polishing pad conditioned by conventional pad conditioning methods suffers from lower material removal rates and non-uniform polishing of the wafer surface. Furthermore, inadequate pad conditioning also shortens the life of the polishing pad. In a typical wafer fabrication facility, where several CMP apparatus are employed, the replacement cost of polishing pads can be significant.
What is therefore needed is an improved apparatus and process for pad conditioning that provides higher material removal rates, uniform polishing of the wafer surface and longer pad life.